try to be compatible to python2.7

Author: Farom

git/repo/base.py

@@ -150,8 +150,9 @@ def __init__(self, path=None, odbt=GitCmdObjectDB, search_parent_directories=Fal
                 # the current working directory is regarded as the top level of your working tree.
                 self._working_tree_dir = os.path.dirname(self.git_dir)
                 if os.environ.get('GIT_COMMON_DIR') is None:
-                    self._working_tree_dir = self.config_reader("repository").get(
-                        'core', 'worktree', fallback=self._working_tree_dir)
+                    gitconf = self.config_reader("repository")
+                    if gitconf.has_option('core', 'worktree'):
+                        self._working_tree_dir = gitconf.get('core', 'worktree')
                 self._working_tree_dir = os.getenv('GIT_WORK_TREE', self._working_tree_dir)
                 break